There are innumerable situations in which a gas requires to be stored for subsequent release under substantially controlled conditions for practical use to be made of the physical and/or chemical properties of the gas. By way of example, stored and released gas may be employed for pressurised dispensing of a substance from a container using the gas as a propellant.
Pressure pack dispensers (commonly but often incorrectly referred to as "aerosol" containers) are employed to dispense a very large number of different substances (hereinafter termed the "product") having a wide range of physical and chemical properties, notably in respect of consistency and viscosity. A pressure pack dispenser is often generally cylindrical usually being fabricated as a sheet metal can, and has a manually-operable valve to control the flow of product from the dispenser. The outflowing product is propelled by a propellant gas stored under pressure in the pressure pack dispenser, the propellant gas being placed in the dispenser at about the same time as the dispenser is loaded with the product to be dispensed. The propellant gas may be unseparated from the product by any mechanical barrier, or the propellant gas may be separated from the product by a barrier which prevents the passage of propellant gas into the produot while simultaneously more or less freely transmitting propellant gas pressure to the product; such a barrier may comprise a flexible impermeable sheet which may be in the form of a bag or alternatively the barrier may comprise a piston slidable within the (conveniently) cylindrical dispenser, for example as described in European Patent Specification EP0089971.
A number of practical considerations limit the substances which can be used as propellant gases and/or the circumstances in which a given substance can be used as a propellant gas. By way of non-limiting examples, such considerations include the ability to sustain pressure within acceptable limits during use, safety factors which include flammability and toxicity of the propellant, and chemical reactivity of the propellant with the container and, mainly in the case of non-barrier dispensers, reactivity of the propellant with the product to be dispensed. By way of a non-limiting example of the circumstances affecting use of a substance as a propellant gas in a non-barrier dispenser, the substance may be substantially inert with respect to one product but react unfavourably with another product (unless isolated by a barrier).
For many years the substances collectively known as CFC's (chlorofluorocarbons) were popular for use as propellants in pressure pack dispensers owing to favourfable pressure characteristics combined with non-flammability and apparent non-toxicity, but CFC's are now perceived as extreme environmental hazards and are the subject of international sanctions; CFC's are no longer acceptable as propellant substances in pressure pack dispensers. Although some readily available gases are free of hazards and are substantially unreactive (for example, nitrogen), gases per se are generally unsuitable for use as propellants in pressure pack dispensers because of unacceptably rapid fall-off of propellant pressure during use of the pressure pack dispenser. Elaborations of construction and use may reduce the unwanted effects of these adverse pressure characteristics, but at the expense of increased complexity and cost, and possibly an increased hazard arising from increased initial internal pressure in the pressure pack dispenser.
Two-phase gas/liquid pressure pack propellant systems may give more acceptable pressure characteristics in terms of an acceptably low fall-off of propellant pressure during use of the pressure pack dispenser, in comparison to a single-phase gas-only system, where the liquid in a two-phase gas/liquid pressure pack propellant system is a pressure-liquefied form of the propellant gas. However the requisite pressure at ambient temperature may be unacceptably high in the context of conventional pressure pack dispensers; additional or alternative disadvantages of two-phase gas/liquefied-gas propellant systems are that they tend to employ gases which are flammable and potential substances of abuse, such as propane, butane and propane/butane mixtures. (It should be noted that such two-phase gas/liquefied gas propellant systems are essentially single-material propellant systems, where the single propellant material is present in both gas and liquid phases; this `single material` nature is not altered by the propellant being a mixture such as butane and propane, since the components of such mixtures change phase together, and a chemically distinct liquid is not present in such systems.)
A further consideration is that even in the case of a pressure pack dispenser with a theoretically perfect barrier such that the propellant gas is supposedly perfectly isolated from the user of the pressure pack dispenser and from the immediate environment at the time of use of the dispenser, unless strict precautions are taken over the ultimate disposal of the spent dispenser (if necessary, with rigorous decontamination), the dispenser will eventually release its contents through corrosion or mechanical damage, hence admitting the propellant to the environment. For this reason, barrier-type pressure packs are not an acceptable solution to long-term environmental problems.
To summarize the main considerations for the adoption of a given propellant system in a pressure pack dispenser, the propellant system should be:
(a) free of toxicity over any length of time and in any feasible concentration;
(b) free of environmental hazard over any length of time;
(c) free of other hazards, including but not restricted to hazards of fire and explosion;
(d) maintain adequate dispensing pressure on the product throughout use of the pressure pack dispenser, without excessive pressure at any time;
(e) at least in non-barrier dispensers, be compatible, and preferably non-reactive, with the product to be dispensed; and
(f) be reasonably economic.
The above list of desiderata for a propellant system is only a general indication and is in no way definitive to the exclusion of any other factors; further, the desiderata are not mutually exclusive in the sense that a characteristic of a selected propellant may satisfy two or more desiderata simultaneously (for example, a hypothetical inert substance may be both non-toxic and non-flammable, as in the case of nitrogen).